Final answer:
In Shakespeare's 'Hamlet,' King Claudius reacts during the 'play within a play' when the scene mirrors his own murderous act, confirming his guilt to Hamlet.
Step-by-step explanation:
The point in the "play within a play" when Claudius reacts is during the performance of 'The Murder of Gonzago,' also known as 'The Mousetrap' in Shakespeare’s 'Hamlet.' As the play depicts a scene similar to the murder of Hamlet's father, King Hamlet, by his uncle Claudius, the current king, Claudius reacts by abruptly rising and leaving the room, thereby interrupting the play.
This reaction confirms Hamlet’s suspicions that Claudius is guilty of the murder. Hamlet cleverly uses the performance to "catch the conscience of the king," using the emotional power of theater to reveal truth.
In the play within a play in Hamlet, Claudius reacts at the point where the Player King is poisoned by pouring poison into his ear. This is the climax of the play within a play and it elicits a strong reaction from Claudius, who recognizes the parallel between the murder in the play and his own crime of killing King Hamlet.